Distillate fuel derived from the Fischer Tropsch process is highly paraffinic and has excellent burning properties and very low sulfur. This makes Fischer Tropsch products ideally suited for fuel use where environmental concerns are important. However, due to their highly paraffinic nature, Fischer Tropsch distillate fuels have problems with poor seal swell properties, and poor lubricity.
The impact of lowering the aromatic content of distillate fuels used as diesel fuel or jet fuel on seal swell in diesel and jet engines is known, and became important when California switched from conventional diesel fuel to Low Aromatics Diesel Fuel (LAD). LAD does not contain zero aromatics, but must contain less than 10%. Literature references to these problems include: Transport Topics, National Newspaper of the Trucking Industry, Alexandria, Va., “Fuel Pump Leaks Tied to Low Sulfur,” Oct. 11, 1993; Oil Express, “EPA's diesel rules leading to shortages, fleet problems, price hikes,” Oct. 11, 1993, p 4; Marin Independent Journal, “Motorists in Marin angry over fuel change,” Nov. 11, 1993, p A1; San Jose Mercury News, “Mechanics finger new diesel fuel,” Dec. 3, 1993; and San Francisco Chronicle, “Problems With New Diesel Fuel, Clean Air, Angry California Drivers,” Dec. 23, 1993.
The swelling of gaskets can be monitored by the use of known tests. For example, a description of test methodology is presented in SAE Paper No. 942018, “Effect of Automotive Gas Oil Composition on Elastomer Behavior,” October 1994, which describes seal swell and hardness changes which were measured in test procedures based as far as possible, on a British Standard (BS) method BS 903 Part A 16 [British Standard Institute, ‘Methods for testing vulcanized rubber,’ Part A 16:1987—Determination of the effect of liquids], which is broadly similar to American Society for Testing and Materials procedures D 471 [Test Method for Rubber Property-Effect of Liquids] and D 2240 [Test Method for Rubber Property-Durometer Hardness]” (See FIG. 12.) The paper examines volume swelling of five types of elastomers: hydrogenated nitrile, low nitrile, medium nitrile and low nitrile rubbers, and fluorocarbon elastomer.
A summary of work carried out to assess problems associated with California low sulfur/low aromatics fuels is presented in the California Governor's “Diesel Fuel Task Force Final Report,” dated Mar. 29, 1996. The report notes results of measurements carried out on O-rings before and after immersion in fuels: volume and weight change by ASTM D 471 [Test Method for Rubber Property-Effect of Liquids], hardness by ASTM D 1415 [Test Method for Rubber Property-International Hardness], and modulus of elasticity, ultimate tensile strength and elongation by ASTM D 1414 [Test Methods for Rubber O-Rings].
Since the transition from conventional distillate fuels to low aromatic fuels created problems with seal swell, greater seal swell problems associated with the transition to a highly paraffinic distillate fuel component made from a Fischer Tropsch process is expected. This may limit the use of Fischer Tropsch distillate fuel.
An additional problem associated with processes that convert Fischer Tropsch products into distillate fuels is that significant quantities of light naphtha are also produced. This light naphtha cannot be blended into most distillate fuels (especially diesel fuel and jet fuel) because it is too volatile. The production of this light naphtha limits the total production of desired distillate fuel. Thus, improvements in the yield of distillate fuel from a Fischer Tropsch process is desired.
There is a need in the art for distillate fuels with acceptable seal swell properties. There is further a need in the art for distillate fuels with satisfactory lubricity properties. Finally, there is a need in the art for distillate fuels with satisfactory properties which can be obtained from Fischer Tropsch process products. This invention provides such distillate fuels and the processes for their manufacture.